This can be a difficult situation. I knew someone whose friend was actually forced into being a hitman against their will. Basically, they had to kill people or they would be killed by the people forcing them to do it.

It's a pretty scary thought. They hated doing it and could barely live with themselves, but they managed to get saved from it.

If nothing else, I suppose at least they were assassinating other gangsters (they weren't one, but were being forced into killing by gangsters) and not innocent people.

I actually faced this scenario. I asked my friend whether or not his target deserved it. When the reply was, "yes." I let it go. Lots of people have killed lots of other people. The world is a large place and there are veterans and former gang members who all-in-all are pretty decent people despite their chosen professions. Rape, child molestation, even vandalism can be so much worse than killing because they involve either malice or a psychological failing. Killing an enemy is merely a part of life in some cultures, including sub-cultures within the US. I'd be far more worried about a drug addicts, people with anger management issues, or generally unstable sorts rather thanmere killers. So long as you aren't talking about serial killers or some additional craziness, don't sweat it.